The invention relates to a method of reducing the flow viscosity of polyacrylates for greater ease of processing in an extruder with simultaneous improvement in the adhesive properties.
In the field of pressure-sensitive adhesive compositions, there is an ongoing demand for new developments. Generally, pressure-sensitive adhesive (PSA) compositions are used for pressure-sensitive adhesive tapes, the adhesive tape comprising a backing and a PSA composition. The uses of adhesive tapes are very diverse. One field of application is the medical segment. In the case of skin plasters, there is direct contact between the PSA composition and the patient""s skin. For these applications, the requirements imposed on the PSA composition are especially stringent, since it is necessary to avoid the instances of skin irritation and allergic reaction that are nevertheless observed with patients in many cases. However, the direct contact of the adhesive composition with the skin may also be utilized, for example, by implementing active substances in the PSA composition. The active substance is then slowly transferred to the patient via the skin.
A further important field is that of applications in the industrial sector. By way of example, double-sided adhesive tapes are used to bond trimstrips on automobiles, to bond electronic chips, or to bond displays in mobile phones. For high-level industrial applications, polyacrylate PSA compositions are preferred on account of their transparency and weathering stability. Besides these advantages, however, they must also meet stringent requirements in respect of shear strength. This is achieved by means of polyacrylates of high molecular weight, with subsequent efficient crosslinking.
As well as the ongoing optimization of the PSA compositions, however, the application techniques for the coating of the backings are also being optimized. As a result of the cost pressure this produces, the hotmelt coatings are increasingly displacing the traditional solvent coating techniques.
The first acrylic hotmelt PSAs were described in patents as long ago as the 1970s. NL 66 06 711 and NL 70 09 629 first described the use of polyacrylates, or polyacrylates and polymethacrylates, as hotmelt PSAs in PSA tapes. The substances processed most commonly were relatively nonpolar polyacrylates with a low acrylic acid fraction and a low flow viscosity. An attempt was made to solve the problem of the production of adhesive compositions of high shear strength from acrylic hotmelts by means of efficient crosslinking on the backing. In DE 27 43 979 A1, for example, benzoin acrylates were copolymerized as comonomers into the acrylic hotmelt and, after coating, were crosslinked directly on the backing using UV radiation. A similar route to the preparation and processing of acrylic hotmelts was shown in U.S. Pat. No. 5,073,611. In that case, benzophenone and acetophenone were incorporated as acrylated photoinitiators into the acrylate polymer chain. This was followed by crosslinking with UV radiation.
A further method for the efficient crosslinking of the acrylic PSAs is the copolymerization of acrylates having electron-donating groups. Such acrylates stabilize free radicals that form in the course of crosslinking, and thus achieve high degrees of crosslinking after appropriate exposure to UV light or electron beams. Examples are tetrahydrofurfuryl acrylates [EP 0 343 467 B1], tertiary amine monomers [WO 96/35725] and tertiary N-butylacrylamide monomer [U.S. Pat. No. 5,194,455].
Likewise used to reduce flow viscosity and improve the processing properties of hotmelt PSAs is polymer blending with block copolymers. Here, preference is given to the use of SIS and SBS systems (SIS: styrene-isoprene-styrene copolymers; SBS: styrene-butadiene-styrene copolymers). WO 95/19393 described, for example, the blending of such block copolymers with polyacrylates, entailing an increase in the tack of these PSAs.
In contrast, there has been no success to date in attempts to process acrylic hotmelts of high molecular mass and high polarity in an extruder without a reduction in molecular weight, in order to obtain, after extrusion coating, a composition of high shear strength.
The high cohesion of polyacrylates used for adhesive compositions may be explained by the high proportion of polar components. The average molecular weight of the adhesive compositions is 1000000 g/mol. The large number of polar fractions results in a high flow viscosity, which hinders processing in the extruder and the subsequent coating of a backing with this polymer. At high temperatures, in fact, the flow viscosity goes down again, but at excessive temperatures there are instances of damage to the polymer. This process leads to an unwanted deterioration in the adhesive properties of the product.
It is an object of the invention to provide a method whereby the flow viscosity of hotmelt pressure-sensitive adhesives may be reduced with a simultaneous improvement in the profile of adhesive properties. The intention is to optimize the processing properties of the hotmelt PSAs by this means, especially for the hotmelt process and in the extruder. Moreover, it should be possible to crosslink the polymer mixture after processing.
This object is achieved by means of a method as described in the main claim. The dependent claims relate to advantageous embodiments of this method and to the use of the polyacrylates processed by this method.
The invention accordingly provides a method of reducing the flow viscosity of a polyacrylate composition, which comprises adding
a) to a polyacrylate, a polyacrylate copolymer, a polyacrylate mixture or a mixture of polyacrylates and polyacrylate copolymers having an average molecular weight of more than 500000 g/mol,
b) a polyacrylate, a polyacrylate copolymer, a polyacrylate mixture or a mixture of polyacrylates and polyacrylate copolymers having an average molecular weight of less than 500000 g/mol, component (b) possessing functional groups crosslinkable by UV irradiation.
Particularly advantageous for this method are a component (a) having an average molecular weight of between 500000 and 4000000 g/mol and/or a component (b) having an average molecular weight between 200000 and 400000 g/mol.
A further development comprises, subsequent to this method, first precrosslinking component (b) by ultraviolet irradiation and in a subsequent step achieving crosslinking of the already precrosslinked component (b) with component (a).
It is advantageous for the crosslinking of component (b) with component (a) to be induced by means of electron beams.
It is appropriate to use, as component (a), polyacrylate copolymers of the following monomers
a1) acrylates and/or methacrylates of the following formula
CH2xe2x95x90CH(R1)(COOR2),
where R1xe2x95x90H or CH3 and R2 is an alkyl chain with 1-20 carbon atoms,
at 75-100% by weight, especially 86-90% by weight, based on component (a),
a2) acrylic acid and/or methacrylic acid of the following formula
CH2xe2x95x90CH(R1)(COOH),
where R1xe2x95x90H or CH3,
at 0-10% by weight, especially 4-6% by weight, based on component (a),
a3) olefinically unsaturated monomers containing functional groups,
at 0-15% by weight, especially 6-8% by weight, based on component (a), at 60-99% by weight, based on the overall polymer blend.
The method proceeds very effectively if component (b) comprises at least one acrylate copolymer with copolymerized photoinitiator at 1-40% by weight, based on the overall polymer blend.
Copolymers of the following composition are, in an outstanding manner, used as component (b):
b1) acrylates and/or methacrylates of the following formula
CH2xe2x95x90CH(R1)(COOR2),
where R1xe2x95x90H or CH3 and R2 is an alkyl chain with 1-20 carbon atoms,
at 70-99.99% by weight, especially 73-99.9% by weight, based on component (b),
b2) acrylic acid and/or methacrylic acid of the following formula
CH2xe2x95x90CH(R1)(COOH),
where R1xe2x95x90H or CH3,
at 0-10% by weight, based on component (b),
b3) olefinically unsaturated monomers containing functional groups, at 0-15% by weight, based on component (b),
b4) photoinitiator functionalized by olefinic double bonds,
at 0.01-5% by weight, especially 0.1-2% by weight, based on component (b), at 1-40% by weight, based on the overall polymer blend.
Furthermore, a particularly advantageous variant of the method comprises a procedure wherein the polymerization of the monomers to give the mixture of the polyacrylates takes place in the presence of component (b).
The polyacrylate prepared by one of these methods is outstandingly suitable for use as a PSA composition. Backing materials that may be used, for adhesive tapes, for example, are materials customary and familiar to the skilled worker, such as sheets (polyester, PET, PE, PP, BOPP, PVC), nonwovens, wovens and woven sheets, and also release paper, if appropriate. This list is not intended to be conclusive.
The invention described is highly suited to achieving the objects described. It presents a polymer blend whose flow viscosity is greatly reduced as compared with that of the PSA composition on which it is based but whose adhesive properties have at the same time been improved by blending together two polyacrylate components of different average molecular weight.
The polyacrylate of higher molecular mass may be, for example, any polymer which has adhesive properties in accordance with the Handbook of Pressure-sensitive Adhesives, p. 172, xc2xa71, 1989. With an average molecular weight of 1000000 g/mol, these adhesive compositions have a high viscosity. Polyacrylates especially suitable for the method described are those having an average molecular weight of between 500000 and 2000000 g/mol.
The viscosity may be reduced by adding a low-viscosity component which acts as a lubricant in the processing step but may subsequently be crosslinked on the backing. For this purpose it is possible and effective to use acrylic copolymers into which a photoinitiator has been copolymerized, which can therefore be activated for a crosslinking reaction by exposure to ultraviolet light. Photoinitiators which may be used in this context are the compounds relevant and known to the skilled worker; the following photoinitiators may be given here by way of example, without wishing to impose any restriction: benzophenones, acrylated or methacrylated benzophenones, benzophenone derivatives, thioxanthones, benzil ketals, xcex1-hydroxyalkyl phenones, xcex1-aminoalkyl phenones, titanocenes, camphorquinones, trichloromethyltriazines and thioxanthenes.
As correspondingly UV-activatable polymers it is possible with a high level of efficiency to use some commercially available products which are used industrially as polymer blending components. Examples thereof include the acResins A 203 UV(copyright) and A 258 UV(copyright) [BASF AG]. These polyacrylates have a molecular weight of around 300000 g/mol and are UV-crosslinkable by virtue of a photoinitiator. Owing to low polar fractions and a relatively low molecular weight, the flow viscosity of these products is relatively low.
The examples below are intended to illustrate the invention without subjecting it to any unnecessary restriction.
Test Methods
The following test methods were used to evaluate the adhesive properties of the PSA compositions prepared.
180xc2x0 Bond Strength Test (Test A)
A 20 mm wide strip of an acrylic PSA composition applied as a film to polyester was applied to steel plates cleaned twice with acetone and once with isopropanol. The PSA strip was pressed onto the substrate twice using a 2 kg weight. The adhesive tape was then immediately peeled from the substrate at 300 mm/min and at an angle of 180xc2x0, and the force required to do this was measured. All measurements were conducted at room temperature.
The results are recorded in N/cm and have been averaged from three measurements.
Shear Strength (Test B)
A 13 mm wide strip of the adhesive tape was applied to a smooth steel surface which was cleaned three times with acetone and once with isopropanol. The area of application was 20*13 mm2 (length*width). The adhesive tape was then pressed onto the steel support four times, applying constant pressure. At 70xc2x0 C., a 0.5 kg weight was fastened to the adhesive tape, and a 1 kg weight at room temperature.
The measured shear stability times are reported in minutes and correspond to the average of three measurements.
Dynamic Mechanical Analysis, DMA (Test C)
The measurements were conducted with the dynamic stress rheometer instrument from Rheometrics. The mechanical loss factor tan xcex4 was monitored as a function of the frequency in an interval from 0.1 to 100 rad/s at 130xc2x0 C. Measurements were carried out with parallel plate arrangement.
Sample Preparation